1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery, more particularly to a non-aqueous electrolyte secondary battery with improved safety and load characteristics.
2. Description of Related Art
Vapor-grown carbon fibers may be produced by thermally decomposing carbon compounds at a temperature of 800.degree.-1300.degree. C. in the presence of ultrafine iron and nickel as catalyst. These vapor-grown carbon fibers are characterized by being easily converted into graphite by heat treatment. For example, graphitized vapor-grown carbon fibers obtained by heat treatment at a temperature exceeding 2800.degree. C. have fewer crystal defects, a network of carbon hexagonal lattices growing tubularly around the axis of a fiber, further similar networks layered on the network outwardly and concentrically, like growth rings of a tree. Therefore, these graphitized vapor-grown carbon fibers are highly strong and elastic, and thermally and electrically conductive.
One of the applications of these graphitized vapor-grown carbon fibers is in a non-aqueous electrolyte secondary battery, in which these carbon fibers are used as an electrode active material.
A non-aqueous electrolyte secondary battery is normally comprised of an anode, separator, cathode and electrolyte. As materials used for the anode, reference may be made to natural graphite, artificial graphite, hardly-graphitizable carbon which is so-called hard carbon, mesocarbon microbeads, pitch carbon fibers and vapor-grown carbon fibers. As materials used for the cathode, reference may be made to a lithium-containing complex oxide such as lithium cobaltate (LiCoO.sub.2), lithium manganate (LiMn.sub.2 O.sub.4, LiMnO.sub.2) and lithium nickelate (LiNiO.sub.2). For the electrolyte there may be used a non-aqueous electrolyte comprising a mixture of a lithium salt and an organic solvent. As the lithium salt reference may be made to LiClO.sub.4, LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6 and LiCF.sub.3 SO.sub.3. As the organic solvent reference may be made to ethylene carbonate (hereinbelow often referred to as EC), propylene carbonate (hereinbelow often referred to as PC), dimethyl carbonate (hereinbelow often referred to as DMC), diethyl carbonate (hereinbelow often referred to as DEC) and methylethyl carbonate (hereinbelow often referred to as MEC).
Recently, non-aqueous electrolyte secondary batteries having excellent cycle properties have attracted attention as large-sized batteries for electric vehicles and domestic electricity storage systems.
In a lithium ion secondary battery, lithium is generally precipitated at the surface of the anode thereof in the form of needles and they sometimes pierce a separator placed between the cathode and the anode when the battery is overcharged, charging current is too heavy, or the like, so that short-circuits can easily be formed. As a result, the lithium ion secondary battery may burst or ignite. Furthermore, overcharging may cause the electrolyte to be decomposed, so that the cycle life of the lithium ion secondary battery may be reduced. On the other hand, overdischarging the battery makes a conductor having an electrode coated with active materials become dissolved, so that the cycle life of the lithium ion secondary battery is highly reduced. In order to avoid these problems, lithium ion secondary batteries are equipped with a safety device for preventing overcharging and overdischarging.
An organic solvent including a linear carbonate which has a low viscosity, e.g. a mixture of a cyclic carbonate such as ethylene carbonate and propylene carbonate with a linear carbonate, has conventionally been employed as the organic solvent for the purpose of improving a lithium ion secondary battery's properties at low temperature and cycle characteristics. Prior art lithium ion secondary batteries using such a solvent as the above-mentioned have a problem in safety. The problem in safety is that the charged batteries are broken by gases formed by decomposition of the solvent in the batteries, and then ignited, because of a high temperature of the lithium ion secondary battery caused by a heavy current when, for example, the above-mentioned short-circuits are formed, artificial short-circuits are formed in a nailing test, or the like. One of the causes is that the linear carbonate has a low boiling point and a high vapor pressure, and another that an exposed area or active reaction area of a fracture of graphite crystal in the anode promotes the decomposition of the solvent, particularly an electrolytic decomposition of propylene carbonate which leads to production of gases, in other words acts a catalyst in the decomposition, and further that if the anode has an insufficient designed capacity, then lithium will be precipitated at the anode.
Prior art lithium ion secondary batteries have unsatisfactory electric conductivity as well as poor cycle characteristics. Therefore, the art has demanded secondary batteries having good cycle characteristics as well as high stability at a high load, i.e., having a high capacity in charging and discharging at a major current.
In order to improve conductivity of the electrodes, particularly the anode, a small amount of materials for improving conductivity has been added to the electrodes. This addition, however, lowers the relative amount of active materials in an electrode, which leads to a decrease in a capacity of the battery. Further, the present inventors found that such addition itself increased unsafeness of the battery. The reason is considered to be as follows. Materials for improving conductivity have a very great specific surface ratio, which differs widely from that of vapor-grown carbon fibers. Therefore addition of a very small amount of the materials to an electrode will increase an average specific surface ratio of the active materials in the electrode.